Many agriculturally important crops are subject to infection by plant viruses. These viruses can seriously damage a crop and drastically reduce its economic value to the grower. This eventually leads to a higher cost of the goods to the ultimate consumer. Attempts to control or prevent infection of a crop by a plant virus have been made, but until recently none have been completely satisfactory. Recently, coat-protein mediated protection of a plant from infection by a virus has proved to be useful and quite satisfactory.
The potato plant is a species that is particularly subject to viral infections. Potato plants are infected by many viruses of economic importance. One of the most serious viral problems in the potato industry is infection of a potato crop with the potato leafroll virus (PLRV). Substantial economic losses are caused when the virus infects a potato crop as infection with PLRV causes a reduction in both quality and yield of the potato crop. Economic losses caused as a result of PLRV infection have been estimated to be approximately 5% of the dollar value of the total potato crop. Current management of PLRV infection of a crop involves the use of insecticides to control the aphids that transmit the virus, but this method of control is expensive and not totally effective. Potato leafroll virus is a member of the luteovirus group and is characterized as a phloem-limited spherical virus containing a positive-sense single-stranded RNA genome. The host range of PLRV is limited to members of the Solanaceae family of which potato, tobacco, tomato and peppers are the primary members. This virus is transmitted in a persistent manner by aphids. The virion capsid protein of PLRV consists of a structural gene product of approximately 23.1 kd (the coat protein) and a read-through product that is approximately 92 kd which is believed to be an aphid transmission helper component and which is believed to be a part of the virion capsid (Bahner et al. 1990).
It has been shown that expression of a plant virus capsid protein (the coat protein) in a plant can confer resistance to the homologous virus and to related viruses. (Abel et al. 1986; Cuozzo et al. 1988; Hemenway et al. 1988; Stark and Beachy 1989; Lawson et al. 1990). In these studies, the resistance to virus disease is expressed as reduced incidence of infection, delayed symptom development, reduced virus replication or titer and slower or no systemic virus movement. Expression of the virus coat protein in these transgenic plants is responsible for the observed effects in the reduction of virus disease by an as yet undetermined mechanism (Powell et al. 1990; van Dun et al. 1988).
A native (wild-type) PLRV coat protein gene has been isolated and transformed into potato plants to yield transgenic potato plants expressing the PLRV coat protein. Subsequent inoculation of the transgenic plants with PLRV resulted in infected plants which contained detectable virus antigen by ELISA, but at a low level. (Kawchuk et al. 1990) Even though the infected plants exhibited resistance to PLRV, the fact that all of the plants became infected is disadvantageous in that it allows for continued transmission of the virus and high levels of resistance could not be expected. Thus, the prior art shows no reduced incidence of infection through use of a wild-type PLRV coat protein DNA sequence in a transgenic potato plant. In other coat protein mediated protection studies, a reduced incidence of infection was an important criterion for determining whether sufficient resistance was achieved. Furthermore, no statistical significance is provided in the prior art to support the assertion that reduced PLRV antigen levels leads to reduced transmission efficiency. The values for PLRV titers in infected transgenic plants disclosed in the prior art are very closely related, yet the allegedly corresponding values for transmission efficiency from these plants are significantly varied. Thus, one can not conclude from the prior art that a significant reduction in virus transmission is obtained from transgenic plant lines expressing a native PLRV coat protein.
There is, therefore, a continuing need in the potato industry for an improved method for controlling infection by PLRV in a potato crop that provides a more effective means of preventing infection by providing transgenic plants that are highly resistant to infection by PLRV and which exhibit a reduced incidence of infection or no infection.